Harmonic suppressor for voltage control device



E. B. HILKER HARMONIC SUPPRESSOR FOR VOLTAGE CONTROL DEVICE Filed Aug. 26, 1957 3 Sheets-Sheet l April 5, 1960 E. B. HlLKER 2,931,970

HARMONIC SUPPRESSOR FOR VOLTAGE CONTROL DEVICE April 5, 1960 Filed Aug. 26, 1957 E. B. HILKER 2,931,970

HARMONIC SUPPRESSOR FOR VOLTAGE CONTROL DEVICE 3 Sheets-Sheet 3 ffmc ryMf-.e- (wmv) @MM MM.;

United States Patent() HARMONIC SUPPRESSOR FOR VOLTAGE CONTROL DEVICE Erwin B. Hilker, deceased, late of St. Louis, Mo., by

Annamary Hilker, administratrix, St. Louis, Mo., assignor to Wagner Electric Corporation, St. Louis, Mo., a corporation of Delaware Application August 26, 1957, Serial No. 680,342 12 Claims. (Cl. 323-75) The present invention relates generally to the voltage control art, and more particularly to a novel harmonic and phase shift suppressor for use with a voltage control device of the type shown and described in copending application Serial No. 429,465, filed May 13,` 1954, and I. S. Malsbary Patent No. 2,892,146, issued June 23, 1959.

The inventions described in the aforementioned appli# cations include means in combination with a transformer for providing a variable compensating or adjusting voltage which is superposed on, or injected into either the input or the output voltage of the transformer so as to maintain the output voltage at substantially the desired value, regardless of changes in the supply voltage within predetermined limits. In the preferred construction, the adjusting voltage is developed in a bridge circuit of four saturable core reactors whose impedances are responsive to selected external conditions, the magnitude and direction of the adjusting voltage being determined by the relative impedance value of the arms of the bridge circuit; for example, the impedances of the reactors forming the bridge circuit may be controlled automatically responsive to the magnitude of the supply voltage. Furthermore, the control of the transformer may be so set as to hold a constant voltage at the load terminals at the transformer station, or at a selected remote location, regardless of the changes of the supply voltage within predetermined limits.

Although the control device referred to hereinabove has operated very satisfactorily, it has been found that under certain load conditions the voltage across the load contains higher harmonics which may interfere with telephonic communication, and, in addition, the phase angle between the supply and output voltages is somewhat greater than desired.

Therefore, it is an object of the present invention, to provide a novel suppressor for use with the aforementioned voltage control device, for maintaining the output voltage substantially free of harmonic components.

Another object is to provide such a suppressor which will maintain the phase shift between the supply voltage and the output voltage small and substantially constant.

A further object is to provide a suppressor which will reduce the maxima or peaks of the voltage waves across the individual reactors of the bridge circuit and which result from the harmonic voltages contained in these voltages.

Further objects and advantages of the present invention will be apparent from the detailed description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is shown.

Briefly, the invention comprises an electro-magnetic impedance connected in parallel with the adjusting voltage within the bridge circuit and which is reduced to a relatively small value when the supply voltage is substantially normal, and which is maintained at a relatively high value when the supply voltage is abnormal.

In the drawings:

Fig. 1 is a schematic wiring diagram illustrating the circuit of a voltage control device and a harmonic suppressor constructed in accordance with the teachings of the present invention,

Fig. 2 is a simplified wiring diagram of the same circuit as shown in Fig. 1 in which the automatic control components of the impedances of the saturable reactors forming the Wheatstone bridge have been removed.

Fig. 3 is a further simplified schematic wiring diagram in which only the minimum A.C. circuit is shown and the control means for adjusting the impedance of the suppressor reactor as well as the impedances of the Wheatstone bridge saturable reactors are omitted.

Fig. 4 comprises two vector diagrams showing the sup- A ply voltage and the primary voltage when the supply voltage is normal, and when it is greater than the primary voltage,

Fig. 5 comprises a set of curves showing the magnitude of the D.C. currents to the saturable core reactors of the Wheatstone bridge and to the saturable core reactor of the suppressor, for different values of supply voltage, and

Fig. 6 comprises a set of curves showing the reactance values of the various saturable core reactors for different values of supply voltage.

Referring to the drawings more particularly by reference numerals, 10 indicates a power transformer, the out-l put of which is to be controlled. The adjusting voltage which is superposed on the voltage of the transformer 10 is developed in a bridge circuit 12 and the magnitude and direction of the adjusting voltage is determined by a con-v trol circuit 14. These components of the circuit are disclosed in the aforementioned copending applications.

The numeral 15 indicates generally a suppressor circuit and control which is the subject matter of the present invention, as used in combination with the aforementioned bridge circuit.

The transformer 10 includes a secondary winding 116 which is connected to a load by leads 17 and 18, a primary winding 20 and a correcting winding 22.

The Wheatstone bridge 'circuit 12 includes four saturable core reactors 24, 26, 28 and 30, each of which comprises an A.C. winding and a D.C. winding which are identified by like numbers with the letters D.C. or A.C. added thereto.l The A.C. windings of the saturable core reactors are connected together at corners 32, 34, 36 and 38.

One end of the primary winding 20 of the transformer is connected to the corner 38 of the bridge by a conductor 40, and the other end thereof is connected to a supply lead 42. One end of the correcting winding 22 is connected to the corner 36 by a conductor 46, and the other end thereof is connected to the opposite corner 32 by a conductor 48. The remaining corner 34 is connected to theother supply lead 44 by a conductor 45.

Turning next to a consideration of the D.C. windings of the reactors, the windings 24DC and 28DC are connected in series circuit and are connected to a first amplifier 50 through conductors 52 and 54 and a rectifier circuit 56. This portion of the circuit will be referred to hereinafter as the first D.C. power source.

' In like manner, the windings 26DC and 30DC are connected in series circuit and are connected to a second amplifier 58 through conductors 60 and 62 land a rectifier circuit 64. This part of the circuit will be referred to hereinafter as the second D.C. power source.

Both the first and second D.C. power sources are energized from the load side of the transformer l0 through conductors 66 and 68 which are connected to the load leads 17 and 18, respectively. The output of each of said two D C. power sources is also responsive to the;

Patented Apr. 5, 19.60

voltage across conductors 66`nd 68 (i.e. the load voltage) asb'y'nieans of the current flowing through conductors 70, 71, 72 and 73, and through a voltage detector 74 and a rectifier circuit 76 which passes a proportionately larger current for each change in. voltagezyifterI the voltage across the` conductors 66 and 68 exceeds a predetermined amount. The construction and operation .of this circuit are de,- scribed in detail in the aforementioned copending appli-r cation Serial No. 497,978.

Turning next to the suppressor circuit and control *15.1 with which we are particularly concerned (Fig. l), this part of the circuit includes a saturabletcore reactor 78 which contains an AC. winding 78AC and a D.C. winding 78D C. One end of the winding 78AC is connected tothe corner 34 of the bridge by the conductor 45, and. the other end thereof is connected to the opposite corner 38 of the bridgeby means of a conductor 80. The wind? ing 78DC in turn is connected to a suppressor control 82 (shown diagrammatically). For ease o f description of the operation, the suppressor control 82 is shown (Fig. 2)v as comprising a manually'operable variable resistor anda battery. Manifestly, the battery can be replaced by `a rectifier circuit receiving its A.C. inpntenergy from the constant AC. voltage appearing across'the leads 17 and` 18 of the output winding of the transformer 10, and. the control can be made automatically responsive` to se lected external conditions. The variousv types of auto-- matic control'circuits which may be nsed with the harmonic suppressor is the subject matter of copending: application Serial No. 687,084, led September 30, "197157. The 1 ).C. excitation of the suppressor reactor is set to aY relatively high value when the supply {A.C1. voltage is aos:

Assuming that the voltage induced in the correcting winding 22' is in the direction so that the right-hand end thereof is at a higher potential than the left-hand end, the corner 32 of the bridge circuit will be at a higher potential than the corner 36.

However, if the D C. currents from the power sources 50'and 58 (Fig. 2) are such that the impedances of all of normal in order to vdecrease the impedanceof Ythe A.C. v

winding of the suppressor reactor. On the other hand, the D.C. excitation of the suppressor reactor is set to a. very low value when the supply voltage is abnormal in order to increase the impedance of the A-C- winding of the suppressor reactor vto a very high value.

U The normal supply .voltagev condition is a specic point in the operational curve C (Fig. 5), where the suppressor circuit 15 is most active to reduce the higher harmonic voltages and the voltage peaks resulting" therefrom across certain points of the reactor, as well as the phase shift betweenV supply voltage and vntput voltage, as will be discussed rnorc in detail hereinafter.

` Operation;

As mentioned hereinabove, the use of the basic c ircuit described in the .aforementioned copending applications resulted in maintaining the voltage V2 across the load leads 17-18 substantially constant regardless of changes Within a predetermined rango in :the suppl-y voltage V1 across the leads 42H14. The operation of this so-called basic circuit is described in the aforementioned contending applications but ,in order .to niore folly yunderstand the importance and value. .of the present invention, it is deemed advisable to refer very briefly to the operation of the basic circuit. Y

Although the .operation can be explained from the standpoint of the primary winding 2,0 havingl ajcertain number of effective ampere turns, depending Yupon whether the ampere tur-ns of the correcting winding 22 arey either aiding or fopposingi the ampere .turns of the winding 2l), it yisv easier tovdescribe the correcting Winding asV-providing a voltage "e (Fig. 3,) appearing across the corners 34 andr 38of the bridge circuit, which is superposedon'or injected into the. voltage (Fig- 3)V so as toaid or opposel it (or do neither), `depending upon the relationship of theimpedancesof the reactors in the bridge, circuit. 1- l Referring to Fig- 3y if we assunto that the supply voltage V1 is normal, if the load A,v /.voltageVais to be normal it is necessary t'naty the suppl-y voltage V1 egual the induced voltage Ef 'ef that the superposed voltage he sinnr ttitieily ille, f

the reactors 24, 26, 28 and 30 are equal (as shown diagrammatically in Fig. 3 by the four arms of the saturable reactor bridge being of equal length), the Voltage drops across the reactors 24 and 30 will be the same; and, if they are in phase, the corners 34 and 38 will be at the same potential, and the adjusting or injected voltage e will vbe zero. Y

On the other hand, if the impressed voltage V1 is above normal, it is necessary for the superposed voltage e to act in a direction to oppose it so that the voltage E will be less than voltage V1. Under these conditions, the D.C.` current supplied by power sources 450 and 58 (Fig. 2) are unequal, whereby the impedance `of the re,- actors r`24 and 28 are less than the impedance of reactors 26 Vand 30.., Consequently, the corner 34 of the bridge circuit is at a higher potential than the corner 38 and the' voltage e is subtracted vectorially from V1. In like manner, if the supply voltage V1 is below normal, the potential of the corner 38 is maintained above the po,- tential of the corner 34, and therefore the voltage fef aids the voltage V1. l

Up to this point it has been assumed that the voltage drops across the bridge reactors are cophasal. These conditions were assumed in order to simplify the theoretical explanation. The assumption of cophasal relations between the voltage drops across the bridge reactors is very nearly satisfied when the current flowing inthe 1211i mary of the transformer is small in comparison with ythe current flowing in the reactors. However, in reality, these cophasal voltage conditions exist only rarely. When the transformer is loaded, the voltages across the different fre-y actors are no longer cophasal and the voltage relations become much more complex. For example, when the four reactors have equal impedance values and the load current flowing through the corners 34 and 38 is cophasal with the voltage across the Vleads 48 and 46, the voltage "e appearing across the corners 34 and 38 of the bridge is approximately at right angles to the voltage across the leads 46 and 48 of the winding 22, or approximately at vand 34 of the bridge attains a nearly cophasal relation,-

ship with respect to the voltage across the leads 46 and 48 of the correcting winding 22, and with respectto the voltage E across the windings 20. This is illustrated in Fig. 4l wherein the path of the end of vectore is shown when the impedances of the bridge reactors are changed, as when the supply voltage varies from below normal to above normal. The path of the end of the vector e is shown as a parabola in Fig. 4 and this is most generally the case as determined by theoretical computations. However, in practice it has been found that the design constants usually are such that the height of the parabola is smaller than shown in Fig. 4 and the path of the end of the vector "e approaches that of a circle. Y

Thus, under the conditions where E equals V1 and the bridge circuit is in balance with the impedances of the various reactors being equal, the superposedv voltage eis not required, but, being present at right angles to"E it results in a phase displacement between E and V1.

onsequently, under conditions where supply voltage V1 1s normal, itis advisable to sconce the magnitude of the voltage e to a minimum, and that is one of the functions of the suppressor reactor 78. If the impedance of the suppressor reactor 78 were reduced to zero, so as to provide a short circuit between corners 34 and 38, there would clearly be no phase displacement between V1 and dEi,

Turning next to a consideration of the form of the reactance curve for the suppressor reactor 78, Fig. 5 shows the manner in which the D.C. currents to the various saturable core reactors vary as a function of the supply voltage V1. Curves A and B are similar to like curves shown in copending application, Serial No. 497,978, and aredependent upon the characteristics of the amplifiers 50 and 58and the voltage detector 74. Curve C shows the most suitable D.C. current values for the suppressor reactor as found by manually changing the excitation of theD.C. winding of the suppressor reactor until the most favorable excitations were determined as functions of the supply voltage.

However, inasmuch as we are primarily interested in changes in the impedance of the various reactors, Fig. 6 shows the reactance curves for the same reactors plotted against percent normal supply voltage V1. Thus, it will be noted that curves A" and B for the two sets of reactors in the bridge circuit cross at 100 percent normal supply voltage, whereby the impedances of both sets of reactors are equal and the Wheatstone bridge is balanced, ite. E equalsV1. Because there is no need for a superposed or injected voltage e under these conditions, the impedance of the suppressor reactor 78 (curve C") at such time is at a minimum.

If the impedance of the suppressor reactor 78 could be decreased to zero at normal supply voltage, there would be, in effect, a short circuit across the corners 34 and 38; the superposed voltage would therefore be zero, and there would be no phase displacement whatsoever between V1 and E This condition is approached very closely by the instant construction. During an actual test, the value of ewas reduced from 7.5% of the load 'voltage to 2.2% thereof using a saturable core reactor of this type. On the other hand, where the supply voltage V1 is above or below normal, the superposed voltage e performs a useful function, and, during those conditions the D.C. excitation of the suppressor reactor 78 is at zero or a very low value (Fig. 5, curve C) so that its A.C. impedance is maintained very high (Fig. 6, curve C).

As previously mentioned, the suppressor reactor 78 also reduces the higher harmonics in the bridge circuit and in the output voltage. The main objection to the presence 0f high harmonics resides in the fact that they interfere with telephonie communication. Also, the amount of interference is dependent upon the magnitude of the harmonic voltage as such and on the frequency of the harmonies (i.e. whether it is 3rd, 4th, 5th or 6th harmonic, and so forth) and not on the magnitude of the harmonic as compared with the total voltage.

Experience has shown that the harmonic voltages appearing across the A.C. windings of the reactors in the bridge, and especially across the corners 34 and 38, reach their maximum when the supply voltage V1 equals the voltage E and these higher harmonics are reflected by transformer action into the voltage induced in the secondary winding of the transformer 10. It is during this condition that the low impedance reactor 78 across the corners 34 and 38 greatly reduces these higher harmonics.

The same holds true for the suppression of higher harmonics in the individual reactors in the bridge circuit. In addition, the use of the suppressor reactor 78 appreciably reduces the peaks of the voltage waves across the individual reactors because they are the result of the high harmonic voltages. For example, in one transformer when the suppressor reactor 78 was not used, an oscilloscope showed that the voltage wave across the individual reactors was rather steep and decidedly distoned with a rather high peak value. When the suppressor reactor 78 was used, the wave form was smoothed out and the peak was only 58% of the voltage peak without the suppressor reactor 78.

It was also learned that the R.V.A. required by the control system using the suppressor reactor were, for most of the load conditions, less than required when the additional reactor was not used. This was especially outsanding when the supply voltage V1 was normal.

During one test, the following results were observed:

Reactor Volts Amps. R.V.A.

24 (and 28) 176 1. 76 310. 26 (and 30) 187 8 150 or (1504-310)X2= 920 R.V.A.

Reactor Volts Amps R.V.A.

24 (and 28) 128 9 115.

88 1. 68 198 0r (l15+63.5)X2+ 198=555 R.V.A.

Thus, when the suppressor reactor 78 was used the R.V.A. was 555 and when it was not used the R.V.A. was 920.

Thus, it is apparent that there has been provided a novel harmonic suppressor which fulfills all of the objects and advantages sought therefor. It maintains the output voltage and the voltage across the individual reactors in the bridge circuit substantially free of higher harmonics which would normally interfere with telephonic communication.

It also reduces the voltage peaks across the individual bridge reactors.

Furthermore, it materially reduces the phase displacement between the impressed voltage and the output voltage when the supply voltage is normal.

In addition, it reduces the reactive kva. consumed by the reactor Wheatstone bridge, and thus the total` reactive kva. input into the transformer circuit is decreased, especially when the supply voltage is normal.

It is to be understood that the foregoing description and the accompanying drawings have been given only by way of illustration and example, and that changes and alterations in the present disclosure, which will be readily apparent to one skilled in the art, are contemplated as within the scope of the present invention which is limited only by the clims which follow.

What is claimed is:

1. In a voltage regulating device containing a main transformer winding and a correcting winding; a Wheatstone type bridge circuit comprising at least four impedances connected together to provide two sets of opposed corners and two sets of opposed impedances; means connecting one side of the main winding to one lead of a set of leads; means connecting one set of opposed corners of the bridge circuit in series circuit between the other side of the main winding and the other lead of said set of leads; means connecting the correcting winding across the other set of corners; and means for varying the magnitude of at leastone set of irnpedances; the improvement which comprises a variable impedance connected across said one set of corners and characterized by the fact that the value of the impedance is at a minimum when the bridge circuit is balanced.

2. In a voltage regulating device containing a main transformer winding and a correcting winding; a Wheatstone type bridge circuit comprising at least four saturable reactors connected together to provide two sets of opposed corners and two sets of opposed reactors; means connecting one side of the main winding to one lead of a set of. leads; means connecting one set of opposed corners of the bridge circuit in series circuit between the other side of the main winding and the other lead of said set of leads; means connecting the correcting windingY across the other set of corners; and means for varying theimpedance ofl atleastV onel set of reactors; therimprovement which' comprises means Vfor suppressing harmonic components inthe device, said 'last named means including an impedance Vconnected across said oney s`et-` of corners of the bridge circuit; and meansy for varying the value of said impedance, to reduce`- Ythe valuey thereof to a minimum when the bridge circuit is substantia'lly' balanced.

3. In a voltage regulating device containing a main transformer winding and a correcting winding; a Wheatstone type bridge circuit comprising atv least four impedances connected together to provideV two sets of opposed cojrners and two sets of opposed'impedances; means connecting one side of the main windingto one lead ofy a set of leads; means connecting one set of opposed corners of the `bridge circuit in seriesY circuit between the other s'i'de ofthe main winding and the other leadof'saidgset of leads; means connecting the correcting winding across lthe other set of corners; and means for varying the' magnitude of atleast one set of impedances; the irnprovementwhich comprises a saturable core reactor connected. across said one set. of corners and including a D.C. winding; and means for varying the magnitude of the current passing through said D.C. winding.

4'. In a voltage regulating device containing a main transformer windingv and a correcting winding; a Wheat- Ystone type bridge circuit comprising at least four saturable core reactors connected together to provide two sets of `opposed corners andv twov sets of opposed reactors; means connecting one side of the main winding to one lead of a set of leads; means connecting one set of opposed corners of the bridge circuit in series circuit between the other side of the main winding and the other lead*v of said set of leads; means connecting the correcting winding across the other set of corners; and means for varying lthe impedance of at least one set of reactors; the improvement which comprises a saturable core reactor connected across said one set of corners and" including a` DC. winding; Yand means for varying the Vmagnitude of the current passing through said D.C. winding so that-the impedance of said last named reactor is at a minimum value when the bridge circuit is balanced and at a higher value when the bridge circuit is in an unbalanced condition.

5. In an electric control device, the combination of power input and output circuits; a bridge circuit comprising at least four saturable core reactors connected together to provide two sets of opposed corners and two sets of opposed reactors; means including one set of opposed corners for connecting said bridge circuit in series circuit between the power input and output circuits; means connecting a source of voltage across the other set of opposed corriers; means for varying the impedance of at least one of said reactors; a variable saturable core reactance device connected across said one set of opposed corners; and means for varying the reactancel of said device so that the reactance thereof is at a minimum value when the bridge is substantiallybalanced and at a higher value when the bridge is in an unbalanced condition.

6. Inan electrical vcontrol system comprising a bridge circuit having input and Voutput terminals, means for supplying a voltage to the input terminals, and means for ycontrolling the balance conditions of the bridge circuit to provide a variablevoltageat the output terminals, the

l improvement comprising a vvariable impedance device connected in a circuit between the output terminals of the bridge, and means for controlling the impedance of said device in accordancewith the balance conditions vof the bridge circuit for reducing the impedance of the device to a minimum value when the bridge is substantially balanced and forlincreasing the impedance of the device when the bridge is in kanunbalanced condition.

output circuit, a power input circuit connectable to a supply source for supplying power to the power output' circuit, a bridgel circuit having input and output terminals, means for supplying a voltage tothe input terminals, means for controlling the balance conditions Yof thebridge circuit to provideY a variable voltage at the output terminals, and means connecting the output terminals of the bridge circuit in series circuit relation with one of said power circuits, the combination therewith of a variable impedance device connected in a circuit between the output terminals of the bridge, and means for controlling the impedance of said devicel in accordance with the balance conditions of the bridge circuit-so'that the impedance of the device is at a minimum value when the bridge is substantially balanced and at a higher value when the bridge is in an unbalanced condition. p

S. In an electrical control system comprising a bridge circuit including bridge input and output terminals, at least two saturable reactors each connected in a. differentarmof the bridge, meansr forfsupplfying a voltagey to the bridge input terminals, and means' for varying the impedance of said reactors to control'the balance conditions of the bridge circuit, the combinationv therewith of an additional saturable reactor having a reactance winding and a control winding, means connecting said reactance winding in a circuit between the output terminals of the bridge, and means includinga source of control current bridge input and output terminals and including at least two saturable core reactors each having a reactance Winding and a control winding, each of said reactance windings being connected in a different arm ofthe bridge circuit, means for impressing a voltage across the bridge input terminals, means connecting the output terminalsl in a circuit in series between-said power input and output circuits, and means for supplying control current tothe control winding of each` of said reactors to control the balance rconditions of the bridge circuit, the combination therewith of means for suppressing harmonic components in the system, said last named means comprising an additional saturable core reactor having a reactance wind ing and a control winding, means connecting the last named reactance winding in a circuit between the output termials of the bridge circuit, and means including a source of control current connected with the last named control winding for varying the impedance of said last named reactance winding so that the impedance thereof is at a minimum when the bridge circuit is Substantially balanced and at a higher value when the bridge circuit is in an unbalanced condition. t

10. In an electrical control system comprising a power output circuit and a power input` circuit connectable to an A.C. source for supplying power-to the power output circuit, a bridge circuit comprising bridge? input and out,

put terminals, at least two saturable core reactors each having an A.C. reactance winding and a D.C. control winding, said reactance windings each connected Yin a different arm of the bridge, means for supplying an A.C. voltage to the input terminals, means connecting the output terminals in a circuit in series with one of said power circuits, and means for supplying D.C. control currentto the control winding of each of said reactors to control the balance lof the bridge circuit, the combination therewith of an additional saturable core reactorY having areactance winding and a control winding, means connecting the last named reactance winding inV a circuit between'` the output terminals of the bridge, means for supplying D.C. current to the last named control winding, and means for controlling the current in'A said last namedV control 71 I-'n an electrical control system comprising a power 75 winding-in accordance with the balance conditions ofthe:

bridge circuit for maintaining the impedance of said last named reactance winding at a minimum when the bridge circuit is substantially balanced and for increasing the impedance thereof to a higher value when the bridge is in an unbalanced condition.

11. In an electrical control system, the combination of a transformer including a main winding and a correcting winding, a bridge circuit comprising bridge input and output circuits, at least two saturable core reactors each hav ing an A.C. reactance winding and a D.C. control winding, each of said reactance windings being connected in a different arm `of the bridge circuit, means connecting said bridge input circuit with said correcting winding, means connecting said bridge output circuit in series with said main winding, means for supplying control current to the control winding of each of said reactors to control the balance of the bridge circuit, and means for suppressing harmonic components in the system, Said last named means including an additional saturable core reactor having an A.C. reactance winding and a D.C. control winding, means connecting the last named reactance winding across the bridge output circuit, and means including a source of D C. control current connected with the lastnamed control winding for varying the impedance of said last named reactance winding so that the impedance thereof is at a relatively low value when the bridge is balanced and at a higher value when the bridge is in an unbalanced condition.

l0 12. In an electrical control system, the combination of a transformer including main primary and secondary windings and a correcting winding, a bridge circuit comprising bridge input and output circuits, at least two.;

saturable core reactors each having an A.C. reactance winding and a D.C. control winding, each of said react# ance windings being connected in a different arm of the bridge circuit, means connecting said input circuit with said correcting winding, means connecting said output circuit in series with one of said main windings, means for supplying D C. current to the D.C. control winding of each of said reactors to control the balance conditions of the bridge circuit, and means for suppressing harmonic components in the system, said last named means including an additional saturable core reactor having an A.C. reactance winding and a D.C. control winding, means connecting the last named reactance winding across said output circuit, and means including a source of D.C. current connected with the last named D.C. winding for varying the impedance of said last named reactance wind' ing so that the impedance thereof is at a minimum value only when the bridge circuit is substantially balanced.

References Cited in the tile of this patent UNITED STATES PATENTS 2,691,130 Ingersoll Oct. 5, 1954 2,763,831 Uiga Sept. 18. 1956 2,815,480 Ruge Dec. 3, 1957 

